A rotating ship's propeller can cause considerable noise and vibrations, and is also subjected to severe cavitation. One step to meet these problems is to mount the propeller so its blades will pass at a distance from the adjacent parts of the ship's hull corresponding to about 25% of the propeller diameter.
That will impose certain restrictions upon the propulsion equipment, which, taking other specifications into account, may mean a reduced propeller diameter and/or an increased rotational speed.